The size saved in c->geometry also includes the titlebars. Thus, for getting the
window content, we have to subtract this from the size.
Before this, the call to xcb_image_get() was failing with a BadMatch error,
because we were asking for an area that is outside of the actual client's
window's geometry.
Signed-off-by: Uli Schlachter <psychon@znc.in>